In vehicular applications, for example belt-driven starter motors/generators, an electric machine with the maximum possible power density is required, which can be produced as cost-effectively as possible. For operation at high speeds (for example, in excess of 20 000 r.p.m.) and a high belt transmission ratio, a flux switching machine (FSwM), for example, with permanent magnets can be appropriate. Above a given transition speed (as in the case of a permanent magnet-excited synchronous machine), the permanent magnet field in a flux switching machine must also be attenuated, if the speed is to be further increased. This has been confirmed by Shen, J.-X., Fei, W.-Z., “Permanent Magnet Flux Switching Machines—Topologies, Analysis and Optimization”, Fourth International Conference on Power Engineering, Energy and Electrical Drives (POWERENG), Istanbul, 13-17 May 2013, wherein reference is made to the requirement for a controllable main field flux in a permanent magnet flux switching machine. However, in known flux switching machines, a sufficient attenuation of the flux generated by the permanent magnets, by the injection of a current into the pole windings, is more difficult to achieve.
In the case of operation in the field shunting range (i.e. operation with reduced torque in excess of a transition speed), the flux switching machine is operated with reduced current strength and a corresponding phase angle. At least in subregions of the stator, the magnetic flux which is generated by the permanent magnets will then predominate over the magnetic flux which is generated by the energization of the pole windings. In consequence, the resulting overall magnetic flux, even at the peak value of the alternating electric current, will not necessarily generate magnetic field lines which are oriented through this pole winding only. Magnetic fluxes are, inter alia, also generated in magnetic leakage paths (e.g. between rotor teeth, via the air gap). This is because in a flux switching machine of conventional design, the magnetic resistance of a magnetic circuit which is routed through the respective permanent magnets but not through the associated pole winding (magnetic short-circuit), at least in subregions of the permanent magnet and at specific rotor positions, is lower than that of other magnetic circuits, which are not only routed through the respective permanent magnet, but also through the associated pole winding. This is described as a stray flux.
Depending upon the rotor position, it is therefore possible that the magnetic field in these subregions of the permanent magnets is not interlinked with the pole winding, but bypasses the associated pole winding (and is thus routed via a magnetic short-circuit, with respect to the pole winding). Consequently, in many cases, the magnetic flux which is generated by the respective permanent magnet cannot be compensated, to the desired extent, by the magnetic flux which is generated by the pole winding. In other words: the magnetic field which is generated by the respective permanent magnet cannot be sufficiently compensated, without further measures, by the magnetic field which is generated by the pole winding. Independently of the short-circuits associated with the rotor position, minor stator-internal magnetic short-circuits can occur which (notwithstanding their detrimental magnetic effect) are accepted under certain circumstances, for example in the interests of improving or simplifying a stator design by means of internal and/or external bars.